Host hydraulics constrain mistletoe resistance to drought-induced embolism

Increasing drought frequency and intensity under climate change are intensifying forest mortality world-wide, yet how biotic interactions modulate plant hydraulic vulnerability remains poorly understood. Mistletoes are obligate stem hemiparasites that operate at low leaf water potentials and sustain high transpiration while relying on host xylem water supply, raising questions about how their hydraulic safety is achieved and constrained. Here, we employed xylem vulnerability curves, vessel and pit anatomy, water-use indicators, and phylogenetically informed models to examine embolism resistance across 41 mistletoe-host species pairs in tropical China. Mistletoes exhibited greater resistance to drought-induced xylem embolism (more negative P₅₀, the water potentials at 50% loss of hydraulic conductivity) but lower hydraulic efficiency than their hosts. Embolism resistance in hosts was associated with vessel and pit traits, whereas these relationships were weaker in mistletoes and became evident mainly after accounting for species-level variation. Notably, host P₅₀ exerted a dominant direct effect on mistletoe P₅₀, whereas mistletoe anatomical traits played a secondary role. These results indicate that mistletoes operate within a shared mistletoe-host hydraulic continuum in which host traits constrain parasite hydraulic safety. By sustaining high transpiration near host-defined limits, mistletoes may amplify host water stress and forest hydraulic vulnerability.